There are zinc base plated steel sheets that are often used as plated steel products excellent in corrosion resistance. The plated steel sheets are used in various manufacturing industries such as the automobile, household electrical appliance and building material industries. Moreover, plated steel products are used in various other fields such as the plated steel wire and hot-dip galvanized steel product fields.
In order to improve the corrosion resistance of such zinc base plated steel products, the present inventors have proposed a hot-dip Zn—Al—Mg—Si coated steel sheet (see, e.g., Japanese Patent Publication No. 3,179,446).
Furthermore, in order to improve the corrosion resistance of zinc base plated steel products, a zinc base plated steel sheet that is made excellent in age-based darkening resistance by adding Ti to the hot-dip Zn—Al coated steel sheet has been proposed (see, e.g., Japanese Unexamined Patent Publication (Kokai) No. 5-125515). However, the problem that the resultant plated steel sheet shows poor surface smoothness and formability was not taken into consideration.
Still furthermore, a zinc-base plated-steel sheet, the appearance of which is made good by adding Ti, B and Si to a hot-dip Zn—Al—Mg coated steel sheet, has been proposed (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 2001-295015). Although Ti and B are added for the purpose of inhibiting the formation and growth of a Zn11Mg2 phase that makes the appearance of the plated steel sheet poor, the proposal neither considers the problem that the addition makes the surface smoothness and formability of the plated steel sheet poor, nor refers to formation of an intermetallic compound.
However, the surface smoothness and formability are not ensured sufficiently for the above plated steel sheets and other disclosed plated sheets.
When the solidification rate of the plating layer is adequately ensured during hot-dip galvanizing, the plating layer solidifies before the Al phase grows significantly. As a result, a problem of the surface smoothness does not arise. However, when the solidification rate of the plating layer is small, the Al phases significantly grow first. Protrusions and recesses are then formed on the plating layer surface, and the resultant plated steel sheet has the problem that the surface smoothness and formability become poor.